The reason for making a static "Landscape" wallpaper is actually done to hide the TB 6 bug in Landscape mode.
For example, if you downloaded any of the BlackBerry Holiday themes RIM themselves released, than you may have noticed that if you used one of your own wallpapers, the top banner in LS mode was still similar to the original wallpaper...
If you develop a theme with a transparent top banner and leave the wallpaper user customizable (in both portrait & landscape), than what happens is when the user changes the wallpaper to their liking, the top banner (in LS mode ONLY) will still be the top part of whatever the original wallpaper was.
Also, there is another reason... If you've downloaded some themes than you may already know this. Some themes that have LS home screen enabled have a big white box on the right side of the screen (in LS mode ONLY) almost like your BlackBerry didn't render the transition from portrait to landscape correctly. The fix? Making a static wallpaper spefically for LS mode.
It's these reasons why Themer's are doing this. Also, you may have noticed a lot of themes don't even have landscape mode enabled, that it takes you directly to the apps screen. Well, the reasons I mentioned above are why!
Personally, I think it sucks because there are quite a few "work-a-rounds" you have to do in order to make sure your theme comes out correctly on a real device. All things you never had to worry about with TB 5. But, with any luck, RIM is working on the updated version of TB and maybe, just maybe (again, with any luck) we'll see a new release of TB after CES next week! <-- probably just wishful thinking haha
Posted from my CrackBerry at wapforums.crackberry.com